It has been a goal of battery engineers and scientists to provide lighter weight and more reliable battery systems, by selection of corrosion resistant materials, active electrochemical couples and combining them in a well engineered system. The goals are often incompatible, as the failure of the materials of construction often frustrate the aims.
Typical examples of the material problems associated with high energy battery systems are the failure of the separator/electrolyte matrix used in the sodium sulfur battery, and the severe corrosion problems experienced in the carbon anode of the zinc bromine battery systems. This causes both systems to be late in development.
Such problems have been evident in the lead acid battery and has concerned the industry for many years. Corrosion of the lead/lead alloy substrates is the major failure mode in the lead acid battery. As a result, the amount of lead used in the lead grid support is much higher than required from an electrical or electrochemical standpoint. Further, additional metal is included to offset the continual attack of the positive grid during charging of the battery. Larger amounts of lead are also used to offset the poor mechanical characteristics of lead as a support structure.
It is well known among those skilled in the art of lead acid battery manufacture that the most vulnerable item in the construction of a battery is the positive battery plate. This is due to the persistent corrosion of the lead/lead alloy grid during the recharge of the system as shown in the following equation (1).
At the positive electrode ##STR1##
Naturally the exposed lead grid will oxidize to lead dioxide also, see equation (2). As the grid ages it suffers stress corrosion, creep and electrochemical attack. Stress corrosion in particular accelerates the failure of the lead grid, and therefore the grid is made from much thicker cross section than is necessary from electrical conductivity considerations, in order to prevent premature failure.
In the prior art various attempts have been made to overcome this problem, for example, plastic frames coated with lead to act as the electrical conductor and to provide a chemically compatible surface have been suggested, see U.S. Pat. No. 3,607,421.
Other attempts included the use of lead coated titanium mesh, the use of oxide dispersion hardened lead, instead of the fault prone lead alloys. Other attempts have used techniques such as the incorporation of glass fiber and carbon fiber composites to strengthen the structure against creep and stress corrosion.
All the attempts have had one common purpose, to improve the performance of the positive plate of the lead acid battery. Many attempts have been directed at efforts to reduce the corrosion of the grid that holds the active material of the battery in the electrolyte, to thus reduce the amount, and therefore weight, of the lead required to do a specific duty.
None of the above techniques are believed to be a total success. The high oxygen overpotential required to recharge lead sulfate severely limits the material choices the electrochemist or metallurgist can make in this system.